LAT-deficient mice have a peripheral blood lymphocyte phenotype characterized by the absence of T cells, but normal B cell and NK cell numbers, T-B+NK. LAT deficiency has not been described in humans, however, of 42 patients with SCID of unknown etiology who have presented to Duke University Medical Center, six have a T-B+NK+ peripheral blood lymphocyte (PBL) phenotype identical to LAT-/- mice. Patients will first be screened for LAT deficiency by immunoblot analysis of peripheral blood mononuclear cell (PBL) lysates. LAT cDNA sequences will also be determined for all patients, even if LAT protein expression appears normal by immunoblot analysis, because any number of immunologically relevant mutations could result in relatively normal gene transcription and translation. Identified LAT mutations will be confirmed by sequence analysis of patient and parental genomic DNA templates in the region of interest. Expression vectors containing putative mutant LAT cDNA will then be constructed and transfected into LAT- deficient Jurkat T cells to evaluate the effect of mutations on T cell receptor signaling. If technically feasible, analysis of IgE receptor signaling in human LAT-deficient mast cells will also be undertaken. The clinical relevance of these studies is underscored by the fact that LAT- deficient mice suffer from SCID and also have abnormal IgE receptor signaling. Hence, it is important to fully understand the role LAT plays in normal T cell receptor and IgE receptor signaling and delineating the perturbations in TCR signal transduction in LAT-deficient humans would prove to be a valuable tool in this regard This knowledge will be important for any future attempts to target mast cell and basophil LAT with specific inhibitors as potential allergic therapeutic regimens.